1. Field of the Invention
This invention relates to LIDAR technology for detecting matter suspended in air, utilized by an aircraft to measure components of matter suspended in air in distant regions ranging from several hundred meters to approximately several tens of kilometers based on the depolarization ratio of laser light, when laser light is irradiated into the atmosphere and the scattered light resulting from scattering of the laser light in the atmosphere is received.
2. Description of the Related Art
Air turbulence is representative of threats to the flight of aircraft posed by meteorological phenomena, but in addition to this there are such dangerous atmospheric states as ice crystals and volcanic ash, and insofar as such phenomena cannot be detected using weather radar, they remain obstacles to flight. The inventors have been engaged in research and development of Doppler LIDAR using laser light as a means of preventing accidents due to air turbulence. (See for example Japanese Patent Application Laid-open No. 2003-14845, “Wind disturbance prediction system”, published on Jan. 15, 2003, and H. Inokuchi, H. Tanaka and T. Ando, “Development of an Onboard Doppler LIDAR for Flight Safety”, AIAA Journal of Aircraft, Vol. 46, No. 4, July-August, 2009.) LIDAR is an acronym for Light Detection and Ranging, which is a detection method using light. Irradiated light rays are scattered by minute aerosols suspended in air, and by receiving this scattered light and measuring the change in frequency (the amount of wavelength change) due to the Doppler effect, the wind velocity is measured; hence this method is called Doppler LIDAR. Using Doppler LIDAR, it is possible to measure the spatial distribution of aerosols, but aerosol components cannot be measured, and so it is not possible to distinguish between components such as water droplets that are not dangerous to the flight of an aircraft, and components such as ice crystals and volcanic ash that are very dangerous. Further, weather radar which has already been utilized in aircraft-mounted mode uses microwaves with wavelengths of several centimeters, and so are not effective for detecting such minute particles as ice crystals and volcanic ash. Observation of the upper atmosphere from ground facilities, and observation from artificial satellites, are possible; but covering the area of an entire flight is not realistic from the standpoint of cost-effectiveness. Japanese Patent Application Laid-open No. 2010-217077, “Alarm display method of remote air flow, and system for the same”, published on Sep. 30, 2010, presents a method for estimating regions in which matter suspended in air exist, but does not include a method for determining the components thereof.
One example in which ice crystals were inferred to be the cause of an aircraft accident is American Eagle Flight 4184, on Oct. 31, 1994. In this accident, the crash of the aircraft was inferred to be due to loss of control of the airframe as a result of ice crystals which were encountered and which adhered to the main wings. Moreover, All Nippon Airways Flight 173 on May 21, 2001, and Japan Airlines Flight 2408 on Sep. 23, 2004, are thought to be accidents due to air turbulence, but if ice crystals in the upper portion of a cumulonimbus cloud had been detected in advance, there was the possibility that air turbulence could have been predicted. In addition, there exist numerous examples in which it is suspected that ice crystals may be related to crash accidents with unknown causes.
In addition to ice crystals, volcanic ash is also extremely dangerous as suspended matter which is a threat to aircraft. It is thought that during British Airways Flight 9 on Jun. 24, 1982, volcanic ash was melted by the heat of the jet engines and adhered to the inside, so that all engines stopped. This was the first instance in which the four engines of a four-engine jet, which is held to be very safe, had all stopped. Thereafter, all engines stopped on Royal Dutch Airlines Flight 867 on Dec. 15, 1989 as well, and in order to prevent a reoccurrence, volcanic ash information centers (Volcanic Ash Advisory Center, International Airways Volcano Watch, and similar) were established in nine places around the world. However, the function of such centers is to monitor volcanic ash, predict dispersion of volcanic ash, and provide a broad range of other information, but not to provide detailed information for individual aircraft. When an engine sucks in volcanic ash, even when an accident does not result, expensive repair costs are incurred, and considerable damage has occurred in Japan as well due to the volcanic fumes from Sakurajima and Miyakejima. In April 2010, a volcano in the south of Iceland erupted, and large amounts of volcanic ash were spewed upward; at high altitudes where the atmosphere is stable, there is the possibility that volcanic ash may remain at a specific height for a long period of time. Because in this case the dangerous region of airspace could not be determined precisely, it was necessary to completely halt flights over Europe as a whole, causing massive economic losses worldwide.
Although lightning strikes of aircraft, which occur regularly, will never result in a major accident, damage to flight equipment frequently occurs. Lightning strikes of aircraft are caused by excessive accumulation of static electricity on the airframe due to collisions with ice crystals, volcanic ash and the like, and so advance detection of ice crystals and volcanic ash can be expected to have an effect in preventing lightning strikes.